This invention pertains to synthetic polyol ester fluids and in particular to those prepared by the transesterification of ethylene-methyl formate telomerization products with polyols having two to about six free hydroxyl groups.
In the years following World War II, the acyl esters of polyhydric alcohols and alkyl esters of dicarboxylic acids were demonstrated to be high performance, synthetic engine lubricants. The former class of esters are most often prepared from a low molecular weight straight chain carboxylic acid containing 3 to 10 carbon atoms and a polyhydric alcohol (polyol) containing no hydrogens on the carbon "beta" to the hydroxyl group. Typical polyols employed are pentaerythritol, dipentaerythritol, trimethylolpropane, neopentyl glycol, and the like. The formation of these lubricant (polyol) esters is typically catalyzed by a variety of acidic compounds; derivatives of titanium (IV) being especially effective. In lieu of the carboxylic acid, its ester derivative can be substituted.
The bulk properties of the polyol ester lubricants, i.e., viscosity, volatility and low temperature flow characteristics are a reflection of molecular weight and shape, size and structure of the acyl group, number of mixed ester components, functionality of the polyol and method of preparing the mixed esters. It is required that the bulk liquid maintain its ability to lubricate various moving parts of the engine over a broad temperature range. The art teaches that various polyol esters of dicarboxylic acids (e.g., adipic acid) and those of moderate molecular weight linear monocarboxylic acids (e.g., octanoic acid) produce lubricants with the desired properties. It is also taught that .alpha.-mono- and .alpha., .alpha.-di-substituted carboxylic acids produce polyol esters which of themselves are inherently less desirable as synthetic lubricants. These acids can, nonetheless, serve as components of a mixed polyol ester which contains both linear and substituted carboxylic acid moieties. In practice, pure acids or mixtures of pure acids are admixed with a polyol or mixture thereof, generally in the presence of a catalyst, and water is removed by distillation as the lubricant ester is formed. The product is treated with water to hydrolyze and remove catalyst. The residual polyol ester is dried and used, in general, without further purification.
In general, fluids meeting the requirements for synthetic lubricants have the following properties:
(1) Wide liquidus range
(2) Range of available viscosities
(3) Low volatility
(4) Low freezing or pour point
(5) High flash point
(6) Good oxidation and thermal stability
(7) Susceptibility to additive treatment for the improvement of properties such as viscosity index, pour point, oxidation stability, metal corrosion resistance, lubrication and wear characteristics, and the ability of the fluid to maintain clean surfaces.
The synthetic polyol ester fluids of this invention are particularly suited to lubricant and hydraulic applications in engines such as gas turbine, Rankine, Sterling, rotary, spark ignition (Otton Cycle) and compression ignition (Diesel) engines of both 4-stroke and 2-stroke cycle designs. Requirements for all of these encompass many of the properties listed above. More specific requirements are outlined below in terms of low-temperature and moderate-temperature applications.
______________________________________ LOW MODERATE PROPERTY TEMPERATURE TEMPERATURE ______________________________________ Viscosity, cSt, at 210.degree. F. 1-10 1-50 at 0.degree. F. 400-2400 2400-100,000 at -40.degree. F. 400-15,000 at -65.degree. F. 2000-25,000 Pour Point, .degree.F. -90 to 0 0 to 60 Flash Point, .degree.F. 200 to 500 300 to 700 ______________________________________
Two primary regimes of rubbing or sliding and rolling motion lubrication are recognized; hydrodynamic and boundary. The hydrodynamic regime involves that component of lubrication that maintains a film separating the moving parts. This depends upon the functional fluid, and particularly the viscosity of the fluid. Furthermore, the viscosity-temperature and viscosity-pressure properties of the fluid play an important role in this lubrication regime. Viscosity-temperature relationships of functional fluids generally are classified according to their extended viscosity index (ASTM D-2270). Ordinarily, an extended viscosity index (V.I..sub.E) of 100 or more is desirable for most hydraulic and engine lubrication requirements.
The boundary component of lubrication predominates when the fluid base fails to provide a separating layer between the moving surfaces being lubricated. Although the base fluid plays a role in boundary lubrication through the processes of surface adsorption and chemical break-down and reaction at the surfaces; i.e., the generation of surface resins, lubrication in this regime normally is dominated by additives that perform also through interfacial physical and chemical reactions. So-called anti-wear, load-carrying, and extreme pressure (EP) additives function almost exclusively by chemical reaction at the surfaces.
The use of polyol esters of alkanoic acids as synthetic lubricants is well known and these lubricants have been used commercially for many years, chiefly in aircraft gas turbine engines such as those described in the United States military specification MIL-L-23699. Basically, this specification requires a product having the following physical characteristics:
______________________________________ Viscosity, cSt. at 210.degree. F. 5-5.5 at 100.degree. F. 25 min. at -40.degree. F. 12,000 max. Pour Point, .degree.F. -65 max. Flash Point, .degree.F. 475 max. ______________________________________
Products meeting the MIL L-23699 requirements, as well as those of commercial gas turbine-powered aircraft are prepared from esters of polyols such as neopentyl glycol (2,2-dimethyl-1,3-propanediol), trimethylolpropane (2-ethyl-2-hydroxymethyl-1,3-propanediol), pentaerythritol(2,2-bis(hydroxymethyl)-1,3-propanediol), dipentaerythritol(bis-[2,2,2-trihydroxymethylethyl]ether) with mixtures of selected straight-chain and branched-chain acids. Similar polyol esters have been proposed and presumably are used in commercial products for automotive engine lubrication.
Acids used in prior art ester lubricants having the neopentyl structure include the common normal and branched-chain monobasic acids as for example, butyric, n-pentanoic, iso-pentanoic, n-hexanoic, various methyl-branched hexanoic acids, and analogous higher acids having up to a total of 20 carbon atoms. For most purposes, acids having more than 10 to 12 carbon atoms are excluded because of the relatively high pour points of their polyol esters. Furthermore, current art teaches the use of mixtures of acids, generally ranging from products having 5 carbons to those containing about 10 carbons. Fluids include those obtained from natural products such as coconut oil, tall oil, castor oil and tallow via fat splitting or by the ozonolysis of unsaturated acids such as oleic or linoleic acids or mixtures of such acids. Acids may also be obtained through synthetic routes which include hydrocarbon oxidation or the oxidation of aldehydes produced by the hydroformylation of alpha-olefins.